Is it possible to use stamp in chop level?

I am trying to use it, I am using a shift chop and I used the stamp expression on the start parameter, it accepts the expression but it doesn`t work on the copy SOP.

What I could be doing worng?

It should work. I've done it a couple of times. Perhaps the reference to the Copy SOP is faulty?

I am going to look better to it, I´ll send a example as soon as I can.

Thanks.